Method for preventing carbon steel from intergranular cracking

ABSTRACT

A method for preventing carbon steel from intergranular cracking utilizing a reducing gas instead of nitrogen gas for a heat treatment of feeder pipes for a pressurized heavy water reactor (PHWR) nuclear power plant. This heat treatment protects carbon from oxidation to prevent the formation of decarburized layers so that the feeder pipes can be prevented from intergranular cracking. In addition, intergranular segregation of impurities is inhibited and intergranular strength of the carbon steel is enhanced. Therefore, no intergranular cracking occurs despite the concentration of stress.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preventing carbon steels from intergranular cracking. More specifically, the present invention relates to a method for preventing feeder pipes for a pressurized heavy water reactor (PHWR) nuclear power plant from intergranular cracking in which a reducing gas is used instead of nitrogen for heat treatment of the feeder pipes to inhibit the oxidation of carbon steels and the resultant formation of decarburized layers.

2. Description of the Related Art

Generally, a pressurized heavy water reactor (PHWR) for nuclear power generation uses natural uranium as a nuclear fuel and heavy water as a moderator. The use of heavy water as a moderator is advantageous in that nuclear fission occurs rapidly even when natural uranium is used as a nuclear fuel, eliminating the need to enrich natural uranium into a commercial nuclear fuel, but has a troublesome problem associated with the collection of heavy water due to an extremely low concentration of heavy water in natural water. Heavy water accounts for about 1/6,500 of natural water. The first heavy water reactor for nuclear power generation was developed in Canada and it is referred to a ‘Canada Deuterium Uranium (CANDU)’ nuclear reactor or a ‘pressurized heavy water type nuclear reactor’. Wolsung nuclear power plant units operating in Korea employ CANDU nuclear reactors. A nuclear reactor tank (calandria) of a CANDU type nuclear power plant includes a plurality of pressure pipes into which a nuclear fuel is inserted in the longitudinal direction and through which heavy water as a heat transfer medium flows. Feeder pipes are connected to an inlet and an outlet of each of the pressure pipes and extend from a common inlet header or linked to a common outlet header. Each of the feeder pipes has a plurality of bent portions whose inner diameter is relatively small and curvature radius is small.

Pipes (e.g., pressure pipes and feeder pipes) other than those used in areas where corrosion is severe in power generation equipments are mostly made of carbon steels. A frequent problem encountered in the power generation equipments is that the iron in the carbon steels is partially dissolved in water flowing through pipes, resulting in thinning of pipe walls. Research has been conducted to solve the problem of pipe wall thinning. Two-phase mixtures are often discharged from the outlets of pressure pipes and cause the feeder pipes to erode and corrode. Research has also been conducted on the prevention of feeder pipes from erosion and corrosion. From 1997 to date, intergranular cracking has also been found in the bent portions of the feeder pipes (outer diameter: 50 mm, thickness: 7 mm) of the CANDU nuclear reactors. The mechanism for the occurrence of intergranular cracking is not clearly understood.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to solve the above problems. It is a purpose of the present invention to provide a method for preventing carbon steel feeder pipes for a pressurized heavy water reactor (PHWR) nuclear power plant from intergranular cracking upon bending. During heat treatment of feeder pipes, a reducing gas atmosphere is used instead of nitrogen to inhibit the surface oxidation and the formation of decarburized layers.

In order to accomplish the above object of the present invention, there is provided a method for preventing carbon steels from intergranular cracking, the method comprising a heat treatment of carbon steels in a mixed reducing gas atmosphere to inhibit the surface formation of a decarburized layer.

In an embodiment, the carbon steel may be a feeder pipe for a PHWR nuclear power plant.

In an embodiment, the mixed reducing gas may contain hydrogen gas.

In an embodiment, the hydrogen gas may be present in an amount of 25 vol % or more, based on the total volume of the mixed reducing gas.

In an embodiment, the heat treatment may be normalizing.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a cross-sectional optical micrograph of a feeder pipe for a PHWR nuclear power plant;

FIG. 2 shows the results of Auger electron spectroscopy obtained from fresh intergranular fracture surfaces after the decarburized feeder pipes for PHWR nuclear power plants were in-situ fractured under a high vacuum at a liquid nitrogen temperature;

FIG. 3 is a graph showing a change in segregation concentration of phosphorus with heat treatment time at 490° C. after decarburization and the corresponding intergranular fracture strength of a decarburized specimen at a liquid nitrogen temperature;

FIG. 4 is an image showing the fractured surface of a feeder pipe having no decarburized layer after the feed pipe was fractured at a liquid nitrogen temperature;

FIG. 5 shows intergranular cracks at the decarburized surface zone of a carbon steel after bending, which were observed under an optical microscope;

FIG. 6 shows cross-sectional optical micrographs of feeder pipes for PHWR nuclear power plants after the feeder pipes were applied a heat treatment in Examples 1 and 2 and Comparative Example 1;

FIG. 7 shows the intergranular crack profile of a feeder pipe for a PHWR nuclear power plant after the feeder pipe was applied a heat treatment and subjected to a slow strain rate tensile test in Example 1;

FIG. 8 shows the intergranular cracks of, a feeder pipe for a PHWR nuclear power plant after the feeder pipe was applied a heat treatment and subjected to a slow strain rate tensile test in Example 2; and

FIG. 9 shows the intergranular cracks of a feeder pipe for a PHWR nuclear power plant after the feeder pipe was applied a heat treatment and subjected to a slow strain rate tensile test in Comparative Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will now be described in detail. Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The embodiments are described below in order to explain the present invention by referring to the figures.

The present invention provides a method for preventing carbon steels from intergranular cracking, the method comprising introducing a mixed reducing gas in heat treatment of carbon steels. The heat treatment of the carbon steels in the presence of a reducing gas inhibits the formation of a decarburized layer as well as the oxidation. As a result, the carbon steels can be prevented from intergranular cracking during bending.

In actual industrial operations, heat treatment is performed under a nitrogen atmosphere to prevent the oxidation of carbon steels. Before heat treatment, nitrogen as an inert gas is introduced into a heat treatment furnace for a long time to flush air present in the furnace and fills the furnace. The air is not completely removed from the furnace. That is, a very small amount of oxygen remains in the furnace. The oxygen reacts with carbon in the steels to form a decarburized layer. The carbon content of the decarburized layer is low, and instead, impurities (i.e. phosphorus) are present at the grain boundaries. The intergranular strength of the carbon steels is weakened due to the presence of impurities, and as a result, the carbon steel is apt to crack.

In contrast, according to the method of the present invention, the heat treatment of carbon steel is performed in the presence of a reducing gas to prevent the oxidation of carbon. This inhibits the formation of a decarburized layer so that the carbon steel can be prevented from intergranular cracking during severe bending. Since the reducing gas itself is oxidized to protect carbon from oxidation, no decarburized layer is formed in the carbon steels.

The mixed reducing gas may contain hydrogen gas. The hydrogen gas may be present in an amount of 25 vol % or more, based on the total volume of the mixed reducing gas. The volume percentage of the hydrogen gas is an amount sufficient to prevent the oxidation of carbon and the formation of a decarburized layer.

The method of the present invention can be applied to feeder pipes for a PHWR nuclear power plant. If feeder pipes for a PHWR nuclear power plant cracks and finally collapses, radioactively contaminated heavy water leaks from the feeder pipes to cause serious radioactive contamination in widespread areas around the nuclear power plant. According to the method of the present invention, the formation of a decarburized layer is inhibited, which protects feeder pipes for a PHWR nuclear power plant from cracking.

Carbon steels are an iron-based metal containing carbon in an amount less than 2%. Low carbon steels, also called ‘mild steel’, contain 0.3% or less of carbon. Feeder pipes for PHWR nuclear power plants are generally made of low carbon steels.

The heat treatment may be normalizing.

Carbon steels undergo heat treatment to have desired microstructures and properties. Normalizing is a heat treatment process in which a carbon steel is heated to a high temperature to form austenite and is then cooled in air. Austenite refers to the face-centered cubic (FCC) phase in the arrangement of atoms. Heating of carbon steels present in a pearlite structure at room temperature converts the ferrite portion to austenite and decomposes carbon present in the form of cementite to be solid-dissolved in the austenite. That is, the two elements of the carbon steel are converted to austenite at elevated temperature. The subsequent cooling converts a portion of the austenite to proeutectroid ferrite and the other portion of the austenite to pearlite, which is a layered structure of ferrite and cementite, below 723° C. This process is referred to as ‘normalizing’.

Generally, feeder pipes for PHWR nuclear power plants experience normalizing. According to the method of the present invention, the formation of a decarburized layer in a feeder pipe for a PHWR nuclear power plant is inhibited by normalizing the feeder pipe in the presence of the mixed reducing gas to protect the feeder pipe from cracking. The normalizing is typically performed at about 900° C.

FIG. 1 is a cross-sectional optical micrograph of a feeder pipe for a PHWR nuclear power plant immediately after normalizing as a heat treatment process. As shown in FIG. 1, a decarburized layer composed of ferrite was formed on the surface of the feeder pipe.

The inner portion of the feeder pipe was found to be composed of two phases, pearlite and ferrite. Carbon incorporated into iron is solid-dissolved and exists in a body centered cubic (BCC) lattice. The resulting structure is called a ‘ferrite structure’. Only an extremely small amount of carbon is solid-dissolved and exists in the ferrite structure. Most of the carbon exists in the form of a carbide, also known as cementite (Fe₃C). Pearlite refers to a structure in which cementite and ferrite are stacked alternately. Pearlite occupies a region separated from ferrite. The higher the carbon content, the broader the pearlite region.

The surface portion of the feeder pipe was found to be composed of ferrite. That is, the carbon content of the surface portion was lower than that of the inner portion, suggesting the formation of the decarburized layer. Decarburization refers to the phenomenon in which carbon is deintercalated from the surface of the carbon steel to form a layer containing less carbon in a position close to the surface. The reason for the carbon deintercalation is because carbon reacts with oxygen (oxidation) in contact with the surface and is then removed in the form of a gas compound. Once the decarburized layer is formed, the hardness and corrosion resistance of the surface are deteriorated. Referring to FIG. 1, the decarburized layer of the feeder pipe had a thickness of about 150 μm.

FIG. 2 shows the results of Auger electron spectroscopy for the fractured surfaces of decarburized feeder pipes for PHWR nuclear power plants and the contents of phosphorus present on the surfaces after the feeder pipes were in-situ fractured under a high vacuum at a liquid nitrogen temperature. The intergranular carbon content of the decarburized layers decreased and phosphorus as an impurity was present in place of the carbon. That is, intergranular segregation of phosphorus was observed. As shown in FIG. 2, intergranular cracking occurred in all surfaces having bulk phosphorus contents of 50, 190 and 920 ppm. From FIG. 2, it can be seen that as the bulk content of phosphorus in the carbon steel pipes increased, the intergranular segregation content of phosphorus content increased.

FIG. 3 is a graph showing a change in segregation concentration of phosphorus with heat treatment time at 490° C. after decarburization and the corresponding intergranular fracture strength of a decarburized specimen at a liquid nitrogen temperature. As shown in FIG. 3, as the amount of intergranularly segregated phosphorus increased, the intergranular strength was greatly reduced. This is because intergranular segregation of phosphorus as an impurity occurred in the decarburized layer to cause a drastic reduction in intergranular strength. As a result, typical intergranular cracking was found.

FIG. 4 is an image showing the fractured surface of a feeder pipe having no decarburized layer after the feed pipe was fractured at a liquid nitrogen temperature. As shown in FIG. 4, intragranular fracture only was observed with no intergranular fracture.

FIG. 5 shows intergranular cracks at the decarburized surface zone of a carbon steel after bending, which were observed under an optical microscope. As shown in FIG. 5, intergranular cracks having a length of about 40-70 μm were detected in the decarburized layer. That is, intergranular segregation of phosphorus as an impurity occurred in the decarburized layer to cause a drastic reduction in intergranular strength. Intergranular cracking appears to occur at the bent portions of the pipe upon bending.

When the feeder pipe having intergranular cracks is used in a PHWR nuclear power plant, iron of the feeder pipe reacts with dissolved oxygen in heavy water to form thin oxide films. In this case, stress concentrated at the crack tip destroys the oxide films. This repeated formation and destruction of oxide films gradually increases the length of intergranular cracks in the feeder pipe.

As described above, carbon reacts with oxygen (oxidation) during normalizing as a heat treatment process to form a decarburized layer on the surface of a feeder pipe, which induces intergranular segregation of phosphorus as an impurity. This intergranular phosphorus segregation may cause intergranular cracking of the feeder pipe upon bending. When the feeder pipe having intergranular cracks is used in a PHWR nuclear power plant, stress is concentrated at the intergranular cracks to promote intergranular cracking in the feeder pipe. In order to prevent the occurrence of intergranular cracking, it is necessary to protect carbon from oxidation by normalizing as a heat treatment process to inhibit the formation of a decarburized layer.

In the method of the present invention, normalizing as a heat treatment process is performed in a mixed reducing gas to inhibit the formation of a decarburized layer resulting from the oxidation of carbon.

Hereinafter, the present invention will be explained in detail with reference to the following examples, including comparative examples. However, these examples are given merely for the purpose of illustration and are not intended to limit the present invention.

EXAMPLES Example 1

After hydrogen was introduced at a rate of 1 l/min for 20 minutes, normalizing as a heat treatment process was applied to a feeder pipe made of low carbon steel for a PHWR nuclear power plant at 900° C. under a hydrogen atmosphere.

Example 2

Normalizing as a heat treatment process was applied to a feeder pipe made of low carbon steel in the same manner as in Example 1 except that a mixed gas of hydrogen and nitrogen (25:75 (v/v)) was introduced.

Comparative Example 1

Heat treatment was applied to feeder pipe made of low carbon steel in the same manner as in Example 1 except that low-purity nitrogen was used instead of hydrogen.

Experimental Example 1

The cross sections of the feeder pipes applied a heat treatment in Examples 1 and 2 and Comparative Example 1 were observed under an optical microscope to confirm whether a decarburized layer was formed or not. The images are shown in FIG. 6.

No decarburized layer was formed in the feeder pipes applied a heat treatment in Examples 1 and 2, whereas a decarburized layer was found in the feeder pipe in Comparative Example 1.

Experimental Example 2

After slow strain rate tensile tests were conducted on the feeder pipes applied a heat treatment in Examples 1 and 2 and Comparative Example 1, intergranular crack profiles were observed using a scanning electron microscope. The images are shown in FIGS. 7, 8 and 9.

The images of FIGS. 7 and 8 shows that typical ductile fracture was observed without any intergranular cracking in the feeder pipes applied a heat treatment in Examples 1 and 2. In contrast, a decarburized layer was formed in the feeder pipe applied a heat treatment in Comparative Example 1 (FIG. 9). FIG. 9 shows an intergranular crack profile in which cracks having a length of 500 μm or more were found at the edges of the decarburized layer.

Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

As apparent from the foregoing, according to the method of the present invention, no decarburized layer is formed despite the presence of a very small amount of oxygen during normalizing of carbon steel and also intergranular segregation of impurities resulting from the formation of a decarburized layer is not observed. This increases the intergranular strength of the carbon steel and prevents the carbon steel from cracking even though stress is applied to the carbon steel. Particularly, heavy water may flow out from feeder pipes of a PHWR nuclear power plant to cause serious radioactive contamination. Therefore, the method of the present invention would give considerable economic effects in industrial applications.

In addition, according to the method of the present invention, carbon steel with desired physical properties can be obtained after heat treatment without any increase in heat treatment cost because the price of hydrogen is comparable to that of nitrogen. Furthermore, according to the method of the present invention, carbon steel can be treated by heat in a simple and continuous manner in conventional heat treatment equipment to which a hydrogen feeding system is additionally attached. 

1. A method for preventing carbon steel from intergranular cracking, the method comprising heat treatment of carbon steel in a mixed reducing gas atmosphere in order to prevent the formation of a decarburized layer in the carbon steel.
 2. The method according to claim 1, wherein the mixed reducing gas contains hydrogen gas.
 3. The method according to claim 2, wherein the hydrogen gas is present in an amount of 25 vol % or more, based on the total volume of the mixed reducing gas.
 4. The method according to claim 1, wherein the heat treatment is normalizing.
 5. The method according to claim 1, wherein the carbon steel is a feeder pipe for a pressurized heavy water reactor (PHWR) nuclear power plant. 